Mechanism of nucleophilic attack by diethylamine on cationic palladium(II) allyl complexes containing α-diimine ligands
Abstract
The reactions of the cationic complexes [Pd(η3-allyl)(N–N′)]ClO4(allyl = 4-methoxycyclohexenyl, allyl or 2-methylallyl; N–N′= 1, 2-bis(imino)ethanes or pyridine-2-carbaldimines) with diethylamine, in the presence of an activated olefin, in chloroform at 25 °C have been studied. They involve a fast equilibrium displacement of the co-ordinated α-diimine to yield [Pd(η3-allyl)(NHEt2)2]+, accompanied by slow nucleophilic attack at the allyl ligand of the [Pd(η3-allyl)(N–N′)]+ substrate producing [Pd(η2-olefin)(N–N′)](olefin = dimethyl fumarate or fumaronitrile) and allyldiethylamine. As shown by the stereochemical course of the reaction with [Pd(1–3-η3-C6H8OMe)(C5H4N-2-CHNC6H4OMe4)]ClO4, the nucleophilic attack takes place on the allyl face opposite the metal. The equilibrium constants for α-diimine displacement have been determined. They are strongly affected by the structure of the N–N′ ligand and decrease in the order RNCH–CHNR RNC(Me)–C(Me)NR ≈ C5H4N-2-CHNR > C5H4N-2-CHNCMe3(R = C6H4OMe-4). Kinetic studies showed that the pseudo-first-order rate constants (kobs) for the slow amination path display both a first- and second-order dependence on the NHEt2 concentration of type kobs=k2[NHEt2]+k2′[NHEt2]2. The k2 term is related to direct bimolecular attack of NHEt2 on the terminal allyl carbon, whereas the k2′ term is ascribed to a parallel nucleophilic attack by a hydrogen-bonded diethylamine dimer arising from amine self-association.